Problem: $ F = \left[\begin{array}{rr}3 & 1 \\ 2 & -1\end{array}\right]$ What are the dimensions of matrix $ F$ ?
Answer: The first dimension is the number of rows in the matrix. How many rows does $ F$ have? The second dimension is the number of columns in the matrix. How many columns does $ F$ have? So, $ F$ is a 2 $\times$ 2 matrix.